Discharge lamps such as super-high pressure discharge lamps may be used as a light source in image display apparatuses such as projectors. In such a super high-pressure discharge lamp, the arc used as a luminescent spot is formed between projections provided to the electrodes. Therefore, it has been proposed that the projections are formed on the tips of the electrodes, and in order for forming the arc originated from the projections, an alternating current having a signal with a frequency lower than a stationary frequency inserted in a signal with the stationary frequency is supplied to the super-high pressure discharge lamp (see, e.g., Japan Patent Publication No. JP-A-2006-59790).
However, even if an alternating current having a signal with a lower frequency inserted is supplied to the discharge lamp, projections with preferable shapes may not be formed depending on the conditions of the electrodes, and flicker caused by migration of the bright spot of the arc might occur.